1. Field of the Invention
This invention relates to a high voltage system for color television receivers, display monitors and projectors with cathode ray tubes, and in particular, a high voltage system which indirectly tracks the ultor or anode voltage of the cathode ray tubes.
2. Description of Related Art
Variations in ultor voltage adversely affect the performance of deflection circuits. The high voltage generator of the ultor voltage exhibits an internal impedance which decreases the ultor voltage as the picture tube draws increasing beam current. Raster breathing is a form of deflection distortion caused by ultor voltage variations of the cathode ray tubes used in color television receivers, display monitors, and projectors. Raster breathing causes the horizontal width of the raster to shrink and expand as the ultor voltage varies. Other manifestations of reduced performance are reduced peak brightness and poor focus at high beam currents.
It is common practice to generate the ultor or high voltage in a flyback transformer which is part of a horizontal deflection circuit. The amplitude of the retrace pulse voltage across a primary winding and across the retrace capacitor is constant because of the regulated supply voltage B+, and the uniform line frequency and retrace time. The retrace pulse voltage can be regarded as a voltage source for the generation of the high voltage because of the high amount of circulating energy during the retrace interval in the deflection circuit and in the primary winding.
High voltage generation and rectification is often accomplished in a diode split arrangement. A secondary high voltage winding is divided into several sections of windings. High voltage diodes are coupled between the winding sections and between an upper terminal of the high voltage winding and the high voltage terminal of the flyback transformer. The retrace voltage is transformed by the turns ratio of the flyback transformer for obtaining a very high secondary retrace pulse voltage, for example 30 kV peak. The high voltage rectifiers conduct during the voltage peaks and charge the capacitor formed by the aquadag of the picture tube to the ultor voltage. The capacitance of the aquadag is typically 1500 pF to 2500 pF, depending upon picture tube size and type. The ultor voltage is loaded by the beam current of the picture tube. The average beam current is typically between 0 and 2 mA with peaks up to 20 mA.
The flyback transformer and, in particular, the split windings are constructed for obtaining a harmonic tuning which leads to ringing in the split windings. Tuning to odd harmonics produces a more square wave shaped high voltage pulse. The result is an increased conduction time of the diodes and, in turn, a lower high voltage source impedance. The high voltage source impedance shows a strong non-linearity at low beam currents, between 0 and 0.5 mA. As a result, the differential decrease of the high voltage is higher at low beam currents than at high beam currents. Deflection sensitivity is a function of ultor voltage and thus, is high voltage dependent. The non-linear high voltage source impedance produces undesirable variations in display performance, such as raster size.
It is known to obtain a sense voltage from the high voltage generator by sampling the voltage across a bleeder resistor. The sense voltage is then supplied to a high voltage control circuit or is used to limit beam current (ABL) to correct for raster breathing via an E-W correction circuit. However, variation in the sense voltage is proportional to the beam current, but not to the high voltage. This results in poor raster breathing correction because the non-linearity of the high voltage is not accounted for by the E-W correction circuit, which is linear, or the high voltage control circuit. This produces inaccurate results in high voltage regulation or E-W correction because the loading of the focus and G2 electrodes are still beam current dependent. Another disadvantage to using the bleeder resistor is that damage to the picture tube may result is there is a poor connection between the bleeder resistor and the high voltage circuit. A second independent bleeder resistor may be used, specifically to sample only the high voltage. However, this approach is expensive and consumes too much power.